Your doctor is officially an idiot. The direction of your uterus does not determine the curling of the string. The direction of your uterus, and even your cervix, doesn't affect you having a posterior fornix. See where I mean? It's on the backside of your cervix, between the external os and the vaginal wall.
Look at this picture:
You want to have the string longer than your external os so you, oh, i don't know... can confirm it's there!!! And when he removes it, it will suck a LOT because he'll have to go into your cervix with something to get the end of the string and pull it out. Did he cut the string at all in the first place? That's the only thing I can think of for a reason it would stick out too straight and wouldn't go into your posterior fornix.
I am so ready to kick some idiot doctors' asses right now.